Housing
What size is your tank? 10g
What temperature is your tank? 76*F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? Glass Catfish

Food
What type of food do you feed your betta fish? Tropical flakes with a pellet or so on the side when the catfish aren't looking and the occasional bloodworm
How often do you feed your betta fish? 2x a day

Maintenance
How often do you perform a water change? Every 3 weeks or so
What percentage of the water do you change when you perform a water change? 50%-75%. It depends.
What type of additives do you add to the water when you perform a water change? Water Conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Symptoms and Treatment
How has your betta fish's appearance changed? Black tips on his blue fins
How has your betta fish's behavior changed? Not at all
When did you start noticing the symptoms? Today when I took a flashlight to check how everyone was doing in there (my room is kind of dark since the ceiling light has been burnt out and we can't seem to get the cover off to change it)
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? 1 year, +/- a few months

Other: He had recently caught his tail on something, ripping a part of it off. I noticed regrowth, but it's black, not blue. Is this normal? Or is it infected?
I can post a picture of him before his fins were ripped, but because of the lack of lighting in my room, I can't take another one without blinding him with the flash.

I'm not an expert, but I think that as long as the fin is regrowing, it's not infected. My betta got terrible fin rot a few months ago and his tail grew back much darker than his normal blue (close to black). It turned blue after awhile, but I can still see a small difference in color. Anyway based on my experience, I think it's normal for their tail to grow back darker at first.

Okay, thanks. I was just inspecting my other fish, who had chewed off his tail, and he had black too, so I wasn't too sure. His fins are much longer now though, even if they are black. I got confused, so thanks so much!

I can't until the light in my room gets fixed. I feel terrible that my betta fish have to live in the dark for the time being, but I guess since we all have to... Anyway, the flash will blind him, so I'll take a picture once the light does get fixed.
Since a lot of the water evaporates, I put roughly 25% of new water in the tank every week. Is this okay or should I take out more water and replace more when I do this?